Summon relies on the subject terms used by providers that contribute records to the Summon service. This means that in Summon there are Library of Congress Subject Headings (LCSH), Medical Subject Headings (MeSH) terms, along with terms from other sources that providers use for applying subject terms to their content.

If multiple providers contribute a record to the Summon service for the same item, Summon preserves the subject terms from the providers and applies the terms to the single record that gets loaded into Summon.

To enhance discoverability of library catalog items, Summon also applies subject terms from other libraries that have cataloged the same items. For example, a book at your institution is discoverable in Summon by a subject term cataloged by another library that also has the book.

The SubjectTerms field is included in Summon's default search fields, and Summon's relevance ranking algorithm puts a high emphasis on matches in the SubjectTerms field. This is especially beneficial for subject searching and topical searching.

Summon's automated query expansion feature uses mappings taken from LCSH and MeSH. These mappings expand queries from variant terms to subject headings (e.g., "heart attack"->"myocardial infarction", "bionanotechnology"->"nanobiotechnology"), allowing for better matches with subject terms conforming to these controlled vocabularies.